U.S. patent number 7,339,381 [Application Number 10/534,008] was granted by the patent office on 2008-03-04 for object sensing.
This patent grant is currently assigned to Koninklijke Philips Electronics, N.V.. Invention is credited to Cornelis Van Berkel.
United States Patent |
7,339,381 |
Van Berkel |
March 4, 2008 |
Object sensing
Abstract
An object sensing system (50), and method, employing electric
field sensing, also known as quasi-electrostatic sensing and which
may be termed cross capacitive sensing, is described. The system
(50) includes at least one electrode arrangement (30), each
electrode arrangement (30) includes one electric field sensing
reception electrode (32) and two electric field sensing
transmission electrodes (34, 36). One of the electric field sensing
transmission electrodes (36) is driven with an alternating voltage
(130) including at least some antiphase portions, for example an
inverted signal, in comparison to an alternating voltage (120) with
which the other electric field sensing transmission electrode (34)
is driven. This improves the spatial precision of object sensing
performed by detecting changes in the current induced in the
electric field reception electrode (32). In one arrangement the two
electric field sensing transmission electrodes (34, 36) are in the
form of annular rings arranged around the electric field sensing
reception electrode (32).
Inventors: |
Van Berkel; Cornelis (Hove,
GB) |
Assignee: |
Koninklijke Philips Electronics,
N.V. (Eindhoven, NL)
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Family
ID: |
9947691 |
Appl.
No.: |
10/534,008 |
Filed: |
October 30, 2003 |
PCT
Filed: |
October 30, 2003 |
PCT No.: |
PCT/IB03/04823 |
371(c)(1),(2),(4) Date: |
February 15, 2006 |
PCT
Pub. No.: |
WO2004/044827 |
PCT
Pub. Date: |
May 27, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20070139049 A1 |
Jun 21, 2007 |
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Foreign Application Priority Data
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Nov 12, 2002 [GB] |
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0226404.2 |
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Current U.S.
Class: |
324/452; 324/662;
340/561; 324/663; 324/457 |
Current CPC
Class: |
G06K
9/0002 (20130101) |
Current International
Class: |
G01N
27/60 (20060101); G01R 27/26 (20060101) |
Field of
Search: |
;324/452,457,662,663
;382/124 ;340/561 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
US. Appl. No. 10/153,261. cited by other.
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Primary Examiner: Nguyen; Vincent Q.
Claims
The invention claimed is:
1. An object sensing system, comprising: an electric field sensing
electrode arrangement, the electrode field sensing electrode
arrangement comprising: a first electrode arranged as an electric
field sensing reception electrode, a second electrode arranged as a
first electric field sensing transmission electrode, and a third
electrode arranged as a second electric field sensing transmission
electrode; driving circuitry arranged to supply a first alternating
voltage to the first electric field sensing transmission electrode
and a second alternating voltage to the second electric field
sensing transmission electrode, the second alternating voltage
comprising at least some antiphase portions compared to the first
alternating voltage; and sensing circuitry arranged to process
currents, induced in the electric field sensing reception electrode
by electric fields generated by the first and second electric field
sensing transmission electrodes when driven by the first and second
alternating voltages respectively, to detect changes in the induced
current due to an object positioned in the electric fields.
2. A system according to claim 1, wherein the second alternating
voltage is an inverted form of the first alternating voltage.
3. A system according to claim 1, wherein the first and second
electric field sensing transmission electrodes are substantially
annular ring shaped, the first electric field sensing transmission
electrode being positioned substantially around the electric field
sensing reception electrode, and the second electric field sensing
transmission electrode being positioned substantially around the
first electric field sensing transmission electrode.
4. A system according to claim 1, wherein the electric field
sensing reception electrode is substantially a block shape, and the
first and second electric field sensing transmission electrodes are
substantially annular shaped, the first electric field sensing
transmission electrode being positioned substantially around the
electric field sensing reception electrode, and the second electric
field sensing transmission electrode being positioned substantially
around the first electric field sensing transmission electrode.
5. A system according to claim 1, wherein the electric field
sensing reception electrode is substantially a circle shape, and
the first and second electric field sensing transmission electrodes
are substantially annular ring shaped, the first electric field
sensing transmission electrode being positioned substantially
around the electric field sensing reception electrode, and the
second electric field sensing transmission electrode being
positioned substantially around the first electric field sensing
transmission electrode.
6. A system according to claim 1, wherein the electrode field
sensing electrode arrangement further comprises one or more further
electrodes arranged as further electric field sensing transmission
electrodes.
7. A system according to claim 1, wherein the system further
comprises one or more further electric field sensing electrode
arrangements, which along with the electric field sensing electrode
arrangement are arranged in a matrix, and corresponding sensing
circuitry.
8. A method of sensing objects, comprising: providing an electric
field sensing electrode arrangement, the electrode field sensing
electrode arrangement comprising: a first electrode arranged as an
electric field sensing reception electrode, a second electrode
arranged as a first electric field sensing transmission electrode,
and a third electrode arranged as a second electric field sensing
transmission electrode; supplying a first alternating voltage to
the first electric field sensing transmission electrode and a
second alternating voltage to the second electric field sensing
transmission electrode, the second alternating voltage comprising
at least some antiphase portions compared to the first alternating
voltage; and processing currents induced in the electric field
sensing reception electrode by electric fields generated by the
first and second electric field sensing transmission electrodes
when driven by the first and second alternating voltages
respectively; and detecting changes in the induced current due to
an object positioned in the electric fields.
9. A method according to claim. 8, wherein the second alternating
voltage is an inverted form of the first alternating voltage.
10. A method according to claim 8, wherein the first and second
electric field sensing transmission electrodes are substantially
annular ring shaped, the first electric field sensing transmission
electrode being positioned substantially around the electric field
sensing reception electrode, and the second electric field sensing
transmission electrode being positioned substantially around the
first electric field sensing transmission electrode.
11. A method according to claim 8, wherein the electric field
sensing reception electrode is substantially a block shape, and the
first and second electric field sensing transmission electrodes are
substantially annular shaped, the first electric field sensing
transmission electrode being positioned substantially around the
electric field sensing reception electrode, and the second electric
field sensing transmission electrode being positioned substantially
around the first electric field sensing transmission electrode.
12. A method according to claim 8, wherein the electric field
sensing reception electrode is substantially a circle shape, and
the first and second electric field sensing transmission electrodes
are substantially annular ring shaped, the first electric field
sensing transmission electrode being positioned substantially
around the electric field sensing reception electrode, and the
second electric field sensing transmission electrode being
positioned substantially around the first electric field sensing
transmission electrode.
13. A method according to claim 8, wherein the electrode field
sensing electrode arrangement further comprises one or more further
electrodes arranged as further electric field sensing transmission
electrodes.
14. A method according to claim 8, wherein the system further
comprises one or more further electric field sensing electrode
arrangements, which along with the electric field sensing electrode
arrangement are arranged in a matrix, and corresponding sensing
circuitry.
Description
The present invention relates to object or feature sensing using
electric field sensing. Electric field sensing is also known as
quasi-electrostatic sensing, and may be termed cross capacitive
sensing. The present invention is particularly suited to, but not
limited to, fingerprint sensing.
Sensing technologies used for object sensing include capacitive
sensing and electric field sensing, also known as
quasi-electrostatic sensing, and which may be termed cross
capacitive sensing. The use of electric field sensing to detect
objects in 3-D space has been known for a long while, and is used
for example in proximity sensors. In nature, the gnathomenu
petersii fish uses electric field sensing to detect objects. In its
very simplest form, capacitive sensing uses just one electrode and
a measurement is made of the load capacitance of that electrode.
This load capacitance is determined by the sum of all the
capacitances between the electrode and all the grounded objects
around the electrode. This is what is done in proximity sensing.
Electric field sensing, which may be termed cross capacitance
sensing, uses two electrodes, and effectively measures the specific
capacitance between the two electrodes. The electrode to which
electric field generating apparatus is connected may be considered
to be an electric field sensing transmission electrode, and the
electrode to which measuring apparatus is connected may be
considered to be an electric field sensing reception electrode. The
first (transmitting) electrode is excited by application of an
alternating voltage. A displacement current is thereby induced in
the second (receiving) electrode due to capacitive coupling between
the electrodes (i.e. effect of electric field lines). If an object
is placed near the electrodes (i.e. in the field lines) some of the
field lines are terminated by the object and the capacitive current
decreases. If the current is monitored, the presence of the object
may be sensed.
U.S. Pat. No. 6,025,726 discloses use of an electric field sensing
arrangement as, inter-alia, a user input device for computer and
other applications. The electric field sensing arrangement senses
the position of a user's finger(s), hand or whole body, depending
on the intended application.
Arrangements such as those disclosed in U.S. Pat. No. 6,025,726 are
limited as to the precision or sensitivity to which the position of
the object may be sensed. The precision available is undesirably
limited compared to a precision that would be desired or required
for some applications, for example fingerprint sensing where
separate sensing of the position of individual ridges of the
fingerprint is required.
The present inventor has realised it would be desirable to provide
an object sensing arrangement, employing electric field sensing,
with a more localised positional sensing than is provided by
conventional arrangements. This has lead to an analysis of the
fundamental limitations of conventional arrangements, in terms of a
sensitivity profile in terms of how the electric field sensing
output falls off with a distance r, the distance r being the
distance from an idealised receiver electrode. The assessment
performed indicates that conventional arrangements, in the presence
of a ground plane behind the transmitting and receiving electrodes,
provide, at best, a sensitivity profile in which the electric field
sensing output falls off with 1/r.sup.6.
In a first aspect, the present invention provides an object or
feature sensing system employing electric field sensing, also known
as quasi-electrostatic sensing and which may be termed cross
capacitive sensing. The system comprises at least one electrode
arrangement, each electrode arrangement comprising one electric
field sensing reception electrode and at least two electric field
sensing transmission electrodes. One (or, in the case of more than
two electric field sensing transmission electrodes, one or more) of
the electric field sensing transmission electrodes is (are) driven
with an alternating voltage comprising at least some antiphase
portions, for example an inverted signal or other form of signal
characterised by comprising opposite polarity, in comparison to an
alternating voltage with which the other electric field sensing
transmission electrode is driven.
This tends to improve the spatial precision of object sensing
performed when detecting changes in the current induced in the
electric field reception electrode due to the presence of an object
or feature.
In a preferred arrangement, the electric field sensing reception
electrode is in the form of a solid or block shape (as opposed, for
example, to an annular ring shape), the electric field sensing
transmission electrodes are in the form of annular rings, and a
first of the electric field sensing transmission electrodes is
positioned around the electric field sensing reception electrode,
and the second electric field sensing transmission electrode is
positioned around the first electric field sensing transmission
electrode (and in the case of a further one or more electric field
sensing transmission electrodes, this or they are positioned around
the respective preceding electric field sensing transmission
electrode, and so on).
In one arrangement the two electric field sensing transmission
electrodes are substantially in the form of annular rings arranged
around the electric field sensing reception electrode which is
substantially in the form of a filled in circle.
In a further aspect, the present invention provides methods of
sensing objects comprising providing the above mentioned items and
using them accordingly.
In particular, the present invention provides a method of sensing
objects comprising providing at least one electrode arrangement,
each electrode arrangement comprising one electric field sensing
reception electrode and at least two electric field sensing
transmission electrodes. The method further comprises driving one
(or, in the case of more than two electric field sensing
transmission electrodes, one or more) of the electric field sensing
transmission electrodes with an alternating voltage comprising at
least some antiphase portions, for example an inverted signal or
other form of signal characterised by comprising opposite polarity,
in comparison to an alternating voltage with which the other
electric field sensing transmission electrode is driven.
Further aspects of the invention are as claimed in the claims.
The above described arrangements and methods tend to provide an
increased extent of localised positional sensing compared to
conventional arrangements. Improved sensitivity profiles tend to be
provided, which in favourable examples may even offer increased
localisation to the extent that the electric field sensing output
falls off with 1/r.sup.12.
Embodiments of the present invention will now be described, by way
of example, with reference to the accompanying drawings, in
which:
FIG. 1 shows a conventional electric field sensing system (not to
scale);
FIG. 2 is a block diagram showing functional modules of a
conventional current sensing circuit of the system of FIG. 1;
FIG. 3 shows an electrode arrangement (not to scale) of an object
sensing system;
FIG. 4 shows the electrode arrangement of FIG. 3 in cross-section
through the line X.sub.1-X.sub.2 of FIG. 3;
FIG. 5 shows an object sensing system;
FIG. 6 illustrates qualitatively different alternating voltages
provided to certain electric field sensing transmission electrodes;
and
FIG. 7 shows results of calculation of an object response curve
indicating a theoretical normalised signal as a function of a
position of an object relative to an electric field sensing
reception electrode for an idealised radially symmetric arrangement
of electric field sensing electrodes.
First, an outline account will be given of the fundamental
operation of a conventional electric field sensing arrangement.
FIG. 1 shows a conventional electric field sensing system 1 (not to
scale) comprising an electric field sensing transmission electrode
2, an electric field sensing reception electrode 4, an alternating
voltage source 6, and a current sensing circuit 8.
The alternating voltage source 6 is connected to the electric field
sensing transmission electrode 2 and the current sensing circuit 8.
The current sensing circuit 8 is separately connected to the
electric field sensing reception electrode 4.
In operation, when an alternating voltage is applied to the
electric field sensing transmission electrode 2, electric field
lines are generated, of which exemplary electric field lines 11,
12, 13 pass through the electric field sensing reception electrode
4. The field lines 11, 12, 13 induce a small alternating current
which is measured by the current sensing circuit 8 (the current
sensing circuit 8 uses a tapped off signal from the alternating
voltage to tie in with the phase of the electric field induced
current, as will be described in more detail below).
When an object 10, is placed in the vicinity of the two electrodes
2, 4, the object terminates those field lines (in the situation
shown in FIG. 1, field lines 11 and 12) that would otherwise pass
through the space occupied by the object 10, thus reducing the
current flowing from the electric field sensing reception electrode
4. Thus the current level measured by the current sensing circuit
may be used as a measure of the presence of an object in the
vicinity of the two electrodes 2, 4.
FIG. 2 is a block diagram showing functional modules of the
conventional current sensing circuit 8. The current sensing circuit
8 comprises an amplifier 20, a multiplier 22 and a low-pass filter
24. These functional modules may be implemented in any suitable
form, for example using the circuit design disclosed in U.S. Pat.
No. 6,025,726, the contents of which are contained herein by
reference.
In operation, the displacement current 26 induced in the electric
field sensing reception electrode 4 is amplified by the amplifier
20 and multiplied by the multiplier 22 with a tapped-off and phase
shifted (by a phase shift module that is not shown) version 27 of
the voltage applied to the electric field sensing transmitting
electrode 2. The tapped-off voltage is phase shifted so as to
render the phase the same as that of the displacement current 26.
Thus, if we assume here that the amplifier 20 is ideal, i.e. does
not introduce any additional phase shifts to the displacement
current 26, then the phase of the tapped-off voltage is shifted
90.degree.. If, in practise, the amplifier 20 does introduce
additional phase shifts to the displacement current 26, then the
phase of the tapped-off voltage is adjusted as required to
accommodate this.
The output from the multiplier 22 is then low-pass filtered to
provide an output signal 28. The output signal 28 is thus a measure
of the current induced in the electric field sensing reception
electrode 4 by the electric field generated by the electric field
sensing transmission electrode 2, and will vary in response to the
object 10, being placed in the vicinity of the electric field
sensing electrodes 2,4. The output signal 28 is then processed by
external electronics (not shown), as required.
FIG. 3 shows an electrode arrangement 30 (not to scale) of an
object sensing system according to a first embodiment of the
present invention. The electrode arrangement comprises an electric
field sensing reception electrode 32, an inner electric field
sensing transmission electrode 34, and an outer electric field
sensing transmission electrode 36. The electric field sensing
reception electrode has a substantially circular shape, (i.e. a
filled in circle).
The inner electric field sensing transmission electrode 34 is of a
substantially annular ring shape around the electric field sensing
reception electrode 32. The outer electric field sensing
transmission electrode 36 is of a substantially annular ring shape
around the inner electric field sensing transmission electrode 34.
Thus the two transmission electrodes 34, 36 are concentric relative
to each other, and also to the electric field sensing reception
electrode 32.
Each of these electrodes is provided with a lead out or contact
portion so that they can be connected to separate parts of the
control circuitry, as follows. The electric field sensing reception
electrode 32 is provided with a reception electrode contact 37. The
inner electric field sensing transmission electrode 34 is provided
with an inner transmission electrode contact 38. The outer electric
field sensing transmission electrode 36 is provided with an outer
transmission electrode contact 39. Breaks are provided in the
annular ring form of each of the electric field sensing
transmission electrodes 32, 34 to enable the contacts 37, 38, 39 to
be lead out of the arrangement. It will be appreciated that other
contact arrangements may be provided instead, for example contacts
may be made using via holes (in which case complete annular rings
may be formed).
For clarity, FIG. 3 is not drawn to scale, in particular with
respect to the relative sizes of the electrodes compared to the
spaces between them and to the widths of the contacts. The sizes of
the elements described will be chosen by the skilled person
according to a number of factors, including fabrication
capabilities, and the typical sizes of the objects intended to be
sensed.
In this embodiment the arrangement is intended to be used for
fingerprint sensing. The dimensions used in this embodiment are as
follows: Radius of the electric field sensing reception electrode
32: 40 .mu.m Inner radius of the inner electric field sensing
transmission electrode 34: 60 .mu.m Outer radius of the inner
electric field sensing transmission electrode 34: 73 .mu.m Inner
radius of the outer electric field sensing transmission electrode
36: 93 .mu.m Outer radius of the outer electric field sensing
transmission electrode 36: 101 .mu.m Annular spaces between the
respective electrodes: 20 .mu.m and 19 .mu.m Widths of the
contacts: 20 .mu.m
FIG. 4 shows the electrode arrangement 30 in cross-section through
the line X.sub.1-X.sub.2 of FIG. 3. As will be understood from the
description associated with FIG. 3, along the cross-section
X.sub.1-X.sub.2 there are two portions of the inner electric field
sensing transmission electrode 34 and two portions of the outer
electric field sensing transmission electrode 36, as shown. Also
shown in cross-section in FIG. 4 is a ground plane 40 provided
beneath the electric field sensing electrodes 32, 34, 36.
The electric field sensing electrodes 32, 34, 36 and the ground
plane 40 may be fabricated in any convenient manner. Here they are
formed by depositing the electric field sensing electrodes 32, 34,
36 on a top side of a glass plate (not shown), with the ground
plane 40 being deposited on the bottom side of the same glass
plate.
FIG. 5 shows an object sensing system 50 according to a first
embodiment of the present invention. The same reference numerals
have been used to describe the same elements as those already
described above. The object sensing system 50 comprises a plurality
of electrode arrangements, each of the type and shape described
above, i.e. comprising a respective electric field sensing
reception electrode 32, inner electric field sensing transmission
electrode 34 and outer electric field sensing transmission
electrode 36 as shown in FIGS. 3 and 4. The respective electrode
arrangements are arranged in a matrix. For clarity, only one such
electrode arrangement 30 is shown in FIG. 5. Further for clarity,
only part of the electric field sensing reception electrode 32 is
shown, and only one of each of the two annular portions (as
described above with reference to FIG. 4) of the inner electric
field sensing transmission electrode 34 and the outer electric
field sensing transmission electrode 36 are shown (i.e. the parts
shown correspond to a cross-section through the line
X.sub.1-X.sub.3 of FIG. 3, rather than the whole of the line
X.sub.1-X.sub.2 of FIG. 3).
As shown in FIG. 5, a glass plate 52 is positioned above the
electrode arrangement 30. Objects to be sensed may are positioned
against or near the glass plate 52. In the example shown in FIG. 5,
a finger tip is pressed against the glass plate 52. One ridge of
the fingerprint profile of the finger tip is shown in FIG. 5
pressed against the glass plate 52 in the vicinity of the electrode
arrangement 30. By virtue of the owner of the finger pressing his
or her whole finger tip on the glass plate 52, other ridges of the
finger tip (not shown) are at the same time pressed against the
glass plate at other locations on the glass plate 52, i.e.
corresponding to other electrode arrangements of the matrix of
electrode arrangements.
The electric field sensing reception electrode 32 and the inner
electric field sensing transmission electrode 34 are coupled to an
alternating voltage source 6 and a current sensing circuit 8 in the
same way as was described above with reference to FIGS. 1 and
2.
The alternating voltage source 6 is also coupled to the inner
electric field sensing transmission electrode 36, but via an
inverter circuit 56.
A ground plane corresponding to the ground plane 40 described above
with reference to FIG. 4 is also included below the electrode
arrangement 30, but for clarity this is not shown in FIG. 5. This
ground plane is preferable for achieving best sharpness of the
positional localisation, but may be omitted if desired.
The operation of the object sensing system 50 will be explained
with reference to FIG. 6, which illustrates qualitatively the
different alternating voltages provided to the inner and outer
electric field sensing transmission electrodes.
FIG. 6 shows a plot 120 of the alternating voltage V.sub.34 output
by the alternating voltage source 6 and applied directly to the
inner electric field sensing transmission electrode 34. In this
embodiment this alternating voltage is a bipolar square wave of
+/-10V and frequency 100 kHz. Indicated in FIG. 6 are the positive
parts 122 and the negative parts 124 of the cycle of the
alternating voltage V.sub.34.
The inverter circuit 56 inverts the alternating voltage provided by
the alternating voltage source 6 and the inverted output is fed to
the outer electric field sensing transmission electrode 36. FIG. 6
shows a plot 130 of this inverted alternating voltage V.sub.36.
Indicated in FIG. 6 are the negative parts 132 and the positive
parts 134 of the cycle of the inverted voltage V.sub.36.
Thus, considering now the inner and outer electric field sensing
transmission electrodes, it is apparent that when the inner
electric field sensing transmission electrode 34 is being driven by
the positive part of 122 of the cycle of the alternating voltage
V.sub.34, the outer electric field sensing transmission electrode
36 is being driven by the negative part 132 of the cycle of the
inverted voltage V.sub.36. Likewise, when the inner electric field
sensing transmission electrode 34 is being driven by the negative
part of 124 of the cycle of the alternating voltage V.sub.34, the
outer electric field sensing transmission electrode 36 is being
driven by the positive part 134 of the cycle of the inverted
voltage V.sub.36. In other words, the two transmission electrodes
are driven with opposite polarity alternating voltages relative to
each other. This may be termed operating in antiphase.
As explained above with reference to FIGS. 1 and 2, the electric
field produced by the inner electric field sensing transmission
electrode 34 due to the application of the alternating voltage
V.sub.34 produces a current 26 output from the electric field
sensing reception electrode 32 which is modified by the fingerprint
ridge 54. This is input to the amplifier 20, multiplied by the
multiplier 22 with a tapped off and phase-shifted version 27 of the
voltage V34, then low-pass filtered to provide the output signal
28.
However, here the electric field produced by the outer electric
field sensing transmission electrode 36 due to the application of
the inverted voltage V.sub.36 also produces a contribution to the
current 26 output from the electric field sensing reception
electrode 32 which is modified by the fingerprint ridge 54. Thus,
during the operation of the object sensing system 50, this
contribution to the current 26 is also being input to the amplifier
20, multiplied by the multiplier 22 with the tapped off and
phase-shifted version 27 of the voltage V34, then low-pass filtered
to provide its contribution to the output signal 28.
The contribution made by inner electric field sensing transmission
electrode 34 to the current 26 provides a positive contribution to
the output signal 28 when multiplied by the multiplier 22.
However, the voltage applied to the outer electric field sensing
electrode 36 is inverted compared to the voltage applied to the
inner electric field sensing electrode 34. Thus, the contribution
made by outer electric field sensing transmission electrode 36 to
the current 26 provides a negative, rather than positive,
contribution to the output signal 28 when multiplied by the
multiplier 22. Thus the effect is for the contribution of the outer
electric field sensing transmission electrode 36 to modify the
contribution due to the inner electric field sensing transmission
electrode 34. This modification is such as to improve the
dependency of the overall output signal 28 on the position of the
object being sensed, in this case the fingerprint ridge 54,
relative to the position of the electric field sensing reception
electrode 32.
Thus, an effect of providing the additional electric field sensing
transmission electrode 36 driven with inverted voltage (compared to
the conventional arrangement shown in FIG. 1) is to provide
increased positional sensing, as the sensing output 28 from the
current sensing circuit 8 due to the field provided by the inner
electric field sensing transmission electrode 34 is modified by the
field provided by the additional outer electric field sensing
transmission electrode 36.
This effect makes use of the property that electrostatic
interaction falls off with distance. Directly above the receive
electrode, the potential (or electric field strength) is dominated
by the voltage on the inner transmission electrode because this
electrode is nearest in all directions. However, moving away from
the centre, the potential (or electric field strength) from the
inner electrode diminishes quicker than the (negative) contribution
from the outer transmitter electrode because the latter is bigger.
This means that the net signal falls off quicker than would
otherwise be the case. Yet further away from the centre (beyond
both electrodes) the contribution from both the inner and other
electrode tends to zero and the overall (net) signal falls to zero.
Hence by appropriate balancing of the contribution from the inner
and outer electrodes, the sensitivity profile can be changed.
Further consideration is now given to the extent of positional
precision offered by theses arrangements, in terms of positions in
an "x-y" plane parallel to the plane of the electric field sensing
electrodes 32, 34, 36 (i.e. at a fixed "z" distance from the plane,
where x, y and z are orthogonal axes).
By use of a mathematical model, an object response curve indicating
a theoretical normalised signal (i.e. change in the output 28 due
to the presence of an object) as a function of the position of the
object relative to the electric field sensing reception electrode
can be calculated for an idealised radially symmetric arrangement.
More particularly, a circular electric field sensing reception
electrode is considered, with two circular concentric rings
providing an inner and outer electric field sensing transmission
electrode respectively.
FIG. 7 shows the results of one such theoretical calculation of
normalised signal against object position, the object position
being defined in relative units. The calculation was performed for
an idealised situation in which:
the radius of the circular electric field sensing reception
electrode=the inner radius of the inner electric field sensing
transmission electrode =1;
the outer radius of the inner electric field sensing transmission
electrode=the inner radius of the outer electric field sensing
transmission electrode=1.5; and
the outer radius of the outer electric field sensing transmission
electrode=2.
For a given set of dimensions, different results for normalised
signal against object position are achieved for different relative
magnitudes of the two voltages applied to the two electric field
sensing transmission electrodes. (Note that although, in the
embodiment described above, the same magnitude of voltage was
applied to both the inner and outer electric field sensing
transmission electrodes (see e.g. FIG. 6), this need not be the
case, and in other embodiments different magnitudes may be applied
using appropriate circuitry provided as part of the inverter
circuit 56 or elsewhere in the overall system, as desired.) FIG. 7
shows the results for three different calculations based on
respective ratios of V.sub.36/V.sub.34 (strictly speaking ratio of
the magnitudes)=0.65, 0.72 and 0.75 as shown in FIG. 7.
The response curves flatten off with decreasing value of
V.sub.36/V.sub.34,(e.g. V.sub.36/V.sub.34=0.65) i.e. in effect the
inner electric field sensing transmission electrode dominates and
the response tends back to the conventional single transmission
electrode arrangement.
As a corollary, the response curves for increasing value of
V.sub.36/V.sub.34,(e.g. V.sub.36/V.sub.34=0.75), are sharper, but
also display a negative dip i.e. in effect the outer electric field
sensing transmission electrode dominates.
Thus the optimum response (in this example) is achieved for
V.sub.36/V.sub.34=0.72, as the curve for this value falls off
sharply with distance but does not include a negative dip. Indeed
the curve for V.sub.36/V.sub.34=0.72 corresponds to a 1/r.sup.12
curve, providing greatly improved positional precision over
conventional arrangements.
Thus different response characteristics can be achieved or
attempted by optimising or otherwise selecting the relative voltage
magnitudes applied to the two electric field sensing transmission
electrodes. Alternatively, this may also be achieved by optimising
or otherwise selecting the dimensions of the various electric field
sensing electrodes accordingly. Yet another alternative is to
select or vary electrode dimensions and voltages in
combination.
In this embodiment a plurality of electrode arrangements 30 are
provided in a matrix. The outputs 28 from each electrode
arrangement 30 are processed in combination in any suitable manner
by further control and processing circuitry (not shown) according
to the object sensing operation being carried out, in this case
fingerprint sensing and analysis.
One advantage of the above described arrangement is that the
sensing takes place without the object needing to be placed
directly against the sensing electrodes. For example, referring to
FIG. 5, the finger can be pressed against the glass plate 52, which
can be separated from the electric field sensing electrodes 32, 34,
36, thus offering these electrodes protection from physical and
corrosive damage.
In the above embodiment, the dimensions of the various electrodes
are as described. Other dimensions may be used as required for
other applications. For example the electric field sensing
reception electrode, the inner transmission electrode and the outer
electric field sensing transmission electrode may all be of
different radii compared to each other, or any two of these may be
the same as each other or all three may be the same. Also, the
absolute sizes may be different compared to those described
above.
In the above embodiment, the inner electric field sensing
transmission electrode and the outer electric field sensing
transmission electrode were in the form of concentric rings around
the centrally positioned circular electric field sensing reception
electrode. However, shapes other than rings may be employed,
arranged in a concentric layout. In other words, the reception
electrode may be any desired block shape (by which is meant a
filled in shape as opposed to annular), e.g. square, rectangle,
triangle, irregular shape etc., with the transmission electrodes
being any desired annular shape around that, the outer transmission
electrode being an annular shape around the inner transmission
electrode which is itself around the reception electrode.
Another possibility is that an electric field sensing reception
electrode, a first electric field sensing transmission electrode
and a second electric field sensing transmission electrode are
provided, but arranged in any convenient arrangement, i.e. not
necessarily concentric. In this case, any suitable layout of
electrodes may be employed, with the drive voltage supplied to the
second electric field sensing transmission electrode being to some
extent (or fully) inverted, of opposite polarity or antiphase
compared to the drive voltage supplied to the first electric field
sensing transmission electrode.
In the above described embodiments one additional electric field
sensing transmission electrode is provided in addition to a first
or conventional electric field sensing transmission electrode. In
other embodiments, further additional electric field sensing
transmission electrodes may be provided to further modify the
positional sensing effect of the output from the first conventional
electric field sensing transmission electrode. These further
additional electric field sensing transmission electrodes are also
driven with a drive voltage that is to some extent (or fully)
inverted, of opposite polarity or antiphase compared to the drive
voltage supplied to the first electric field sensing transmission
electrode. In the case of annular arrangements, these further
electric field sensing transmission electrodes may be in the form
of further annular shapes. For example, in the case where the first
and second electric field sensing transmission electrodes are
annular rings, these further electric field sensing transmission
electrodes are preferably further annular rings. Also, for any of
these possibilities, one or more further electric field sensing
transmission electrodes may be driven with the same drive voltage
as the drive voltage supplied to the first electric field sensing
transmission electrode.
In the above described embodiment, the form of the drive voltages
is a described with reference to FIG. 6. In other embodiments,
other values and/or forms may be used. For example, other
amplitudes may be used for either or both of the electric field
sensing transmission electrodes. Likewise, other frequencies may be
used. In the above embodiments, the waveform is a square wave.
Other alternating waveforms may be used, for example sine wave.
In the above embodiments, the drive voltage supplied to the second
electric field sensing transmission electrode is to some extent (or
fully) inverted, of opposite polarity, or is antiphase, compared to
the drive voltage supplied to the first electric field sensing
transmission electrode. In the above embodiments, this is in the
sense of each oscillation of the alternating cycle of the drive
voltage that is applied to the first electric field sensing
transmission electrode being replicated in inverse form in the
drive voltage applied to the second electric field sensing
transmission electrode. However it is not necessary that these
drive voltages are applied simultaneously to the electrodes. The
low pass filter 24 has an integrating time constant and a more
general possibility is that within that time constant, both
transmission electrodes are driven an equal amount of time, with
each having the correct (phase or anti phase) relation to the
reference signal 27. To take this point even further, yet another
possibility is that the electrodes are driven in sequence with the
same phase, but that the phase of the reference signal is altered
when the outer electrode is driven, by switching in an additional
phase delay.
In the above embodiments current sensing circuit 28 is employed to
sense the displacement current induced in the electric field
sensing reception electrode. However, any other suitable circuit or
arrangement may be used for this. One possibility is to use current
sensing circuits of a type forming the subject matter of a pending
patent application of the present applicant, i.e. U.S. application
Ser. No. 10/153261 with applicant's reference PHGB010089, the
subject matter of which is incorporated herein by reference.
In the above embodiments the object sensing system is used as part
of a fingerprint sensing and analysing system. However, the present
invention is not limited to such use, and may be employed as a
stand-alone object sensing system or as part of any other process
or system incorporating object sensing.
Furthermore, the term "object" is not limited to discrete objects
that need to be sensed in full, rather this term encompasses
features or elements of a larger object that may be individually
resolved or sensed, for example one or more ridges of a fingerprint
of a finger, as described in the main embodiment above. Another
possibility is that by sensing a particular feature of a given
object, the orientation of the given object may be derivable.
The object sensing system may be incorporated in, or with, a
display device or system, to provide a user input or interaction
means. For example, the object sensing system may be incorporated
on the inside of an active matrix liquid crystal display device,
constructed and operated for example along the lines of the liquid
crystal display device disclosed in U.S. Pat. No. 5,130,829, the
contents of which are contained herein by reference. In this case,
the above mentioned current sensing circuits of the type forming
the subject matter of pending U.S. application Ser. No. 10/153261,
incorporated herein by reference, may be used to particular
benefit.
From reading the present disclosure, other variations and
modifications will be apparent to persons skilled in the art. Such
variations and modifications may involve equivalent and other
features which are already known in the art, and which may be used
instead of or in addition to features already described herein.
Although Claims have been formulated in this Application to
particular combinations of features, it should be understood that
the scope of the disclosure of the present invention also includes
any novel feature or any novel combination of features disclosed
herein either explicitly or implicitly or any generalisation
thereof, whether or not it relates to the same invention as
presently claimed in any Claim and whether or not it mitigates any
or all of the same technical problems as does the present
invention.
Features which are described in the context of separate embodiments
may also be provided in combination in a single embodiment.
Conversely, various features which are, for brevity, described in
the context of a single embodiment, may also be provided separately
or in any suitable subcombination. The Applicants hereby give
notice that new Claims may be formulated to such features and/or
combinations of such features during the prosecution of the present
Application or of any further Application derived therefrom.
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